Recently, a hybrid vehicle and an electric vehicle are attracting attention as ecologically friendly vehicles. A hybrid vehicle has, as power sources, a DC power source, an inverter and a motor driven by the inverter, in addition to a conventional engine. Specifically, power is obtained by driving the engine and, in addition, the DC voltage from the DC power source is converted to AC voltage by the inverter, the motor is rotated by the converted AC voltage, and whereby power is obtained.
An electric vehicle has a DC power source, an inverter and a motor driven by the inverter, as power sources.
In a motor drive device mounted on a hybrid vehicle or an electric vehicle as such, typically, when a malfunction such as short-circuit of a switching element forming the inverter is detected, the inverter operation is stopped, in order to prevent excessive heating of the switching element caused by an excessive current flowing to the short-circuited switching element.
At this time, a back electromotive force generates in a motor coupled to the drive shaft of the vehicle in accordance with the speed of rotation. Therefore, when the motor speed is high, a current passing through the inverter may undesirably increase, as it receives high back electromotive force. Therefore, in some type of vehicles, in response to a detection of inverter malfunction, a clutch arranged between the drive shaft and the motor is disconnected, to stop power transmission from the drive shaft to the motor, and the vehicle enters a so-called limp mode, in which the vehicle runs to a place not to block foot or vehicle traffic, with the clutch disconnected.
At this time, the limp mode running depends only on the inertia acting on the driving wheels, as the power is not supplied to the drive shaft. Therefore, it is difficult to secure a running distance to reliably move the vehicle to a safe area.
In view of the foregoing, recently, a technique has been proposed for ensuring torque necessary for the limp mode running. For example, Japanese Patent Laying-Open No. 2004-120883 discloses an inverter for driving a three-phase AC motor in which an operation of a three-phase AC motor can be continued even if a switching element forming the inverter fails.
According to this laid-open application, the inverter for driving three-phase AC motor includes a DC power supply circuit with a rectifying circuit rectifying an output of an AC power supply, an inverter circuit formed by first to third arm circuits connected in parallel, each including two semiconductor switching elements connected in series and converting a DC voltage from the DC power supply to a three-phase AC voltage, and a PWM controller for PWM (Pulse Width Modulation) control of the inverter circuit, and drives a three-phase AC motor having star-connected excitation windings of three-phases.
In the inverter, the DC power supply circuit has a voltage dividing circuit equally dividing the output voltage of the rectifying circuit and outputting to a neutral point, and the first to third arm circuits have first to third output points formed by a node of two semiconductor switching elements. Between the neutral point and the first to third output points, a neutral point connecting switch circuit is provided, for selectively connecting the neutral point to any one of the first to third output points.
In the structure described above, if any of the plurality of semiconductor switching elements forming the inverter circuit is found defective, the neutral point connection switch circuit connects the neutral point and the output point of the arm circuit including the failed semiconductor switching element. As a result, the excitation windings of three-phase AC motor attain to a state equivalent to a state in which normal two-phase excitation windings are V-connected. In this state, the PWM controller performs PWM-control of four semiconductor switching elements included in two arm circuits regulating the current flowing through the V-connected two-phase excitation windings, and generates an output current of three-phase equilibrium, for driving the AC motor.
Further, Japanese Patent Laying-Open No. 9-23501 discloses an electric vehicle controller in which a failure diagnosis circuit as auxiliary motor control means is used for executing motor drive control in place of a motor control circuit, if a defect is found in any of three current sensors detecting currents flowing through respective phases of a three-phase motor or in a current control circuit executing feedback control based on the detected current from the current sensors.
According to this technique, as the failure diagnosis circuit as the spare motor drive control means, a back-up microcomputer is provided, separate from a microcomputer functioning as the original motor control circuit.
According to the inverter for driving three-phase AC motor disclosed in Japanese Patent Laying-Open No. 2004-120883, in order to continue operation of the three-phase AC motor even after the failure of a semiconductor switching element is found, a neutral point connection switching circuit becomes necessary, for connecting the output point of the arm circuit including the short-circuited semiconductor switching element to the neutral point of the DC power supply circuit. This inevitably leads to larger size of the inverter. Further, it leads to increased cost of the device.
Further, in the electric vehicle controller in accordance with Japanese Patent Laying-Open No. 9-23501, control circuits are provided corresponding to the normal state and abnormal state of the current sensor. Therefore, as in the technique of Japanese Patent Laying-Open No. 2004-120883, it involves the problems of the size and cost of the device. Other patent documents do not disclose any measure to control driving of a short-circuited inverter only by an existing device structure.
The present invention was made to solve these problems and its object is to provide a motor drive device that can ensure safety and output performance of a motor when a failure of an inverter is detected, with a simple and inexpensive device structure.